1. Field of the Invention
The present invention relates to load sensors used for vehicle seats, and particularly, to a load sensor that converts straining of strain-sensor elements to weight values.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-330522 discloses a conventional load sensor. FIGS. 8 and 9 illustrate the structure of such a load sensor. Specifically, FIG. 8 is a side view of the load sensor and FIG. 9 is a partial plan view of the load sensor. Such a load sensor includes a flat-plate resilient member 110 having a stationary segment 111 and a free segment 112; strain-sensor elements 130 to 133 disposed on the same surface of the resilient member 110; and a lever 140 extending towards the center of the resilient member 110. When a load is applied to the lever 140, the resilient member 110 bends and the strain-sensor elements 130 to 133 thus become strained. The load sensor converts the straining of the strain-sensor elements 130 to 133 to weight values.
Referring to FIGS. 8 and 9, the resilient member 110 is a metallic plate formed of, for example, stainless steel and is provided with the stationary segment 111 and the free segment 112. Moreover, the width of the central portion of the resilient member 110 is narrower than the other portions, and the resilient member 110 has resiliency. Two opposite surfaces of the stationary segment 111 are respectively provided with washers 120 and 121, and two opposite surfaces of the free segment 112 are respectively provided with washers 122 and 123. The washers 120 and 121 are fixed to the stationary segment 111 with a screw 143, and the washers 122 and 123 are fixed to the free segment 112 with a screw 144. The lever 140 is disposed on one of the surfaces of the washer 122 and is fixed to the free segment 112. Furthermore, the resilient member 110 is fixed to a base 145 such that the free segment 112 can be strained freely with respect to the stationary segment 111.
All of the strain-sensor elements 130 to 133 are disposed on the undersurface of the resilient member 110 such that a pair of strain-sensor elements 130 and 133 and a pair of strain sensor elements 131 and 132 have the narrow central portion of the resilient member 110 therebetween. When the strain-sensor elements 130 to 133 receive stress, such as a compressive force or tension, the density of a conductive material included in the strain-sensor elements 130 to 133 changes. Accordingly, the resistance of the strain-sensor elements 130 to 133 is variable. Also, the resistances of all the strain-sensor elements 130 to 133 are uniform with one another. The strain-sensor elements 130 to 133 are connected with an input electrode, a ground electrode, a first output electrode, and a second output electrode so as to define a bridge circuit.
An operation of such a conventional load sensor will now be described. When there is no stress applied to the lever 140, the resistance of the strain-sensor elements 130 to 133 is constant and the bridge circuit is in equilibrium. When a load is applied to the lever 140, a compressive force is applied to the stationary segment 111 such that a tension is generated in the free segment 112. This forces the resilient member 110 to bend. Consequently, the strain-sensor elements 130 to 133 become compressed and/or strained such that the resistance of each of the strain-sensor elements 130 to 133 changes with respect to the load applied to the lever 140. Thus, a voltage applied to the bridge circuit through the input electrode is divided by the strain-sensor elements 130 to 133 so as to create a difference in voltage between the first output electrode and the second output electrode. The load sensor then converts this difference in voltage to a weight value via a specific circuit and sends a signal corresponding to the value to a display device (not shown in the drawings).
According to the structure of such a conventional load sensor, when a drop impact test is performed based on the assumption that a load is applied to a vehicle seat during its installation, the maximum stress created by an impact is generated in the stationary segment instead of the narrow central portion of the resilient member. For this reason, since the resilient member is symmetrical with respect to the narrow central portion, an impact applied to the resilient member may lead to different permanent strain between the left side and the right side of the resilient member. This is problematic in that a proper detection output cannot be achieved.